1.0 Field of the Invention
The present invention relates to an Electronic Support Measure (ESM) system and, more particularly, to a circuit for removing interference signals from the frequency spectrum of the RF signals incoming to and being analyzed by the ESM system.
2.0 Description Related to the Prior Art
Electronic Support Measure (ESM) system, such as a prior art ESM system 10 shown in FIG. 1 herein, often employ a single, Wide Bandwidth Receiver 12, for measuring RF frequency parameters on RF signals received via an Omnidirectional antenna 14. This single serial arrangement is in parallel with a multi-channel receiver, such as Crystal Video Receiver (CVR) 16, operatively cooperating with a Constant Beamwidth Lens (CBL) antenna 18, providing, in a manner to be described, quantities 20 comprising RF amplitude, pulse width, Time Of Arrival (TOA), and Direction Finding (D/F) data associated with the incoming RF signal.
In operation, an incoming RF signal entering the Omnidirectional antenna 14 is detected and the RF signal frequency parameters thereof are estimated and sent, by way of the Wide Band Receiver 12, to the System Digital Processor 22. Simultaneously, an adjacent Constant Beamwidth Lens (CBL) Antenna 18 receives the same RF input. The CBL antenna 18, as well as the Crystal Video Receiver (CVR) 16, is comprised of multiple receiving elements. The multiple elements, as well as RF signal portions, are identified herein by the use of subscripts. When appropriate, a grouping of elements is simply referred to by its generic reference number. Each portion 181, 182 . . . 18M of the RF signal received by CBL antenna 18 is delivered to a single Constant Bandwidth Lens (CBL) 24 which focuses the received RF signal, dependent on the azimuth of the RF source relative to the receiving elements, to multiple parallel lens RF outputs 261, 262 . . . 26N. Each lens RF output 261, 262 . . . 26N serving as channels is provided for by the Crystal Video Receiver (CVR) 16, consisting of an RF band filters 281, 282 . . . 28N, protective RF Limiters 301, 302 . . . 30N, and Detector Log Video Amplifiers (DLVA) 321, 322 . . . 32N. Following RF filtering appropriate to the RF band of interest, the DLVA 32 logarithmically detects the input RF signal amplitude and each CVR channel respectively produces an analog video voltage output 341, 342 . . . 34N proportional to the logarithm of the RF input signal envelope power to be further described hereinafter with reference to FIGS. 3 and 4. To permit processing of a wide range of RF input duty cycles, (including CW) outputs of the CVR elements 321, 322 . . . 32N are DC coupled.
The multiple parallel video outputs from the CVR elements 321, 322 . . . 32N on signal paths 341, 342 . . . 34N are provided to a Digitizer/Angle Encoder 36, where each RF amplitude video input is digitized. Multiple adjacent digitized video inputs on signal paths 341, 342 . . . 34N are compared with each other to determine the relative azimuth of the RF signal source, using a relative amplitude comparison process. The highest level RF video input on signal paths 341, 342 . . . 34N is processed to estimate the RF input signal power level, the RF input signal pulse width, and the TOA. This parametric data derived by the Digitizer/Angle Encoder 36, are sent to the System Digital Processor 22 where these data are combined with the RF frequency data received from wide band receiver 12 and then analyzed by the System Digital Processor 22 to determine the type of RF signal source and relative azimuth of the signal source generating the incoming RF signal.
The ESM System 10 of FIG. 1, particularly suited for shipboard use, is a typical design. In practice, the Omnidirectional antenna 14 and the Wide Band Receiver 12 usually provides hemispherical coverage, with two systems employed, each covering one side of a ship. Similarly, the CBL Antenna 18 usually provides instantaneous coverage of a single quadrant, with two CBL Antennas 18 and two Crystal Video Receiver 16 per one Omnidirectional antenna 14 and Wide Band Receiver 12.
The number of CVR 16 channels per CBL Antenna 18 depends on the RF frequency band coverage associated with the incoming RF signals and the desired azimuth measurement resolution; typically, this varies from ten to twenty CVR element grouping per CBL Antenna 18. For example, a CVR 16 having twenty (20) channels requires band filters 281, 282 . . . 2820. The instantaneous RF frequency coverage of a system is usually restricted to a 3:1 bandwidth, for example, 2-6 GHz or 6-18 GHz. A full shipset, covering 2-18 GHz, might then require four Omnidirectional antennas 14 and Wide Band Receivers 12 (one per ship side per band); and each Omnidirectional antenna 14 and Wide Band Receiver 12 would require two Crystal Video Receivers 16 (one per quadrant). Each Crystal Video Receiver 16 would require (typically) fifteen groups of elements 28, 30, and 32. The total elements 28, 30 and 32, per ship, is approximately one hundred and twenty units.
The ESM System 10 has generally proven to be accurate and cost effective. There is, however, a problem with local interference, particularly with own-ship CW emitters (such as CW target illuminators and SATCOM related signals). More particularly, an own-ship CW signal existing in the ESM System 10 operating band and received above the ESM System 10 operating threshold, set for its receiving elements, will be detected and processed by both the single serial arrangement of the Omnidirectional antenna 14 and Wide Band Receiver 12 and the parallel operating CBL antenna 18 and the CVR 16. Any other received RF signal that is of lesser RF power at these antennas 14 or 18 will be obscured by this own-ship signal; if the other signal is associated with a threat, a serious condition occurs, that is, where an own-ship signal has blinded the ESM System 10 to the existence of the threat.
Normally, own-ship interfering signals, such as those generated by CW emitters, are eliminated with the use of fixed or tunable RF notch filters. This approach is useful and effective for the single serial arrangement of the Omnidirectional antenna 14 and Wide Band Receiver 12, but is not useful for the parallel operating CBL antenna 18 and CVR 16. More particularly, consider that a typical installation employs only four serial arrangements of the Omnidirectional antenna 14 and Wide Band Receiver 12, but requires one hundred twenty grouping of CVR 16 elements 28, 30, and 32 to support the parallel operation of the CBL antenna 18 and CVR 16. Further, if multiple signals are present and one signal frequency is suppressed in the single serial arrangement of the Omnidirectional antenna 14 and Wide Band Receiver 12, but not in the parallel operating CBL antenna 18 and CVR 16, the system is likely to produce RF frequency data from one emitter and RF amplitude, TOA, pulse width, and AOA data from a different emitter. Solutions to this problem have, to date, focused on trapping out the interference in the single serial arrangement of the Omnidirectional antenna 14 and Wide Band Receiver 12, reducing the sensitivity of both the Wide Band Receiver 12 and the CVR 16, and shielding the CBL Antennas 18 from the interference. None of these solutions or combinations therefore has proved satisfactory. It is desired to provide an ESM System that reduces or even eliminates the operational detrimental local interference effects created by own-ship emitters, while minimizing the additional circuitry needed to accomplish the elimination.
It is an object of the present invention to provide an ESM system successfully operated on a ship and that reduces or even eliminates the operational detrimental local interference effects created by own-ships emitters, while minimizing the additional logic thereto needed to accomplish the elimination.
It is a further object of the present invention to provide a circuit for reducing the interference suppression and arranging the circuit in the single serial arrangement of the Omnidirectional antenna of the ESM system and minimizing the logic added to the parallel operating the CBL antenna and CVR.
It is a further object of the present invention to selectively desensitize the operation of the parallel operating CBL antenna and CVR arrangement by selectively suppressing one or more RF input frequencies handled by the single arrangement of the Omnidirectional antenna and associated Wide Band Receiver.
Another object of the present invention to cause the operation of the single serial arrangement of the Omnidirectional antenna and Wide Band Receiver to translate the received RF spectrum of the incoming RF signals by a fixed offset frequency and then using that offset frequency, to desensitize the operation of a parallel arrangement of a CBL antenna and CVR.
It is an additional object of the present invention to selectively desensitize the operation of the parallel channels of the CBL antenna and CVR.
It is still a further object of the present invention to synchronize the single serial arrangement of the Omnidirectional antenna and Wide Band Receiver with the operation of the parallel functioning CBL antenna and CVR.
Moreover, it is an object of the present invention to provide an ESM system that eliminates or substantially reduces the analog operating components thereof and replaces those components with a digital operating processor to provide for the handling of digital data.
The invention is directed to an interference suppression circuit and method of operation thereof for multi-channel receivers and is particularly suited for an ESM system.
The Electronic Support Measure (ESM) system has a Constant Beamwidth Lens (CBL) antenna that receives incoming RF signals and provides a plurality of output signals. The ESM system comprises a) a log video amplifier receiver having multiple channels with each channel receiving a respective one of the plurality of output signals of the CBL antenna. The log video amplifier receiver comprises a first mixer having first and second inputs and an output, and with the first input receiving a signal representatively of the respectively output signal of the CBL antenna. The output of the first mixer is connected to a logarithmic amplifier, which provides an output video signal comprising the output signal of the respective channel of the log video amplifier receiver. The ESM system further comprises b) an Omnidirectional antenna receiving the incoming RF signals and providing a respective output thereof; c) a notch filter receiving the output signal of the Omnidirectional antenna and providing a filtered output thereof; d) a Wide Band Receiver receiving the output of the notch filter and providing first and second outputs each representative of the output of the notch filter; e) an oscillator having a predetermined frequency and providing an output signal; and f) a second mixer having first and second inputs and an output, and with the first input receiving the first output of the Wide Band Receiver and the second input receiving the output of the oscillator. The output of the second mixer is connected to the second input of each of the first mixers. The ESM system further comprises g) a digitizer/angle encoder connected to receive each of the output signals of each of the channels of the log video amplifier receiver. The digitizer/angle encoder provides output quantities representative of the amplitude, pulse width and time of arrival (TOA) of the RF signal and direction finding (D/F) data thereof. The ESM system further comprises h) a system digital processor connected to receive the output quantities of said digitizer/angle encoder and said second output of said Wide Band Receiver.